Prior aerial drop penetrometers achieved uncontrolled high velocities during impact or required parachutes to slow their decent. High velocity impact resulted in penetrometer deceleration that was primarily caused by soil inertia, not soil strength. Soil inertia is a function of soil density while soil strength determines soil trafficability and foundation support properties. Parachute dropped penetrometers tended to drift off target and may not impact vertically. An uncontrolled high velocity penetrometer (as used in the prior art) will decelerate upon impact with soil primarily as a result of soil inertia (density) with only a slight affect due to soil strength thereby only providing crude knowledge of the soil strength.
For an aerial-dropped penetrometer that impacts soil, the factors that cause its deceleration include soil strength, friction between the soil and penetrometer surface, and soil inertia (acceleration of the soil mass by the penetrometer). Where reduced complexity is important and soil strength is the parameter of interest, the single most important sensor for an aerial-dropped penetrometer is an accelerometer, which measures the deceleration of the penetrometer upon impact in the soil. Penetrometer deceleration factors can be assessed in terms of friction, strength and inertia using a physical model of penetration impact. In some cases, penetrometer data can be interpreted to determine soil type (when impact velocity is relatively low or both high velocity and low velocity impact velocity is available). At higher impact velocities, both the effects of strain rate (for cohesive soils) and inertia need to be considered.
Soil type can generally be categorized as granular (e.g., sand) or cohesive (e.g., clay). Granular soils obtain their strength through frictional contacts between soil grains that produce an increasing strength with depth as the weight of soil overburden increases. For granular soils, penetrometer deceleration will increase with depth of penetration. Cohesive soils gain their strength primarily through electrical bond attraction between fine soil particles that results in a relatively constant strength throughout the layer of similar soil type. For cohesive soils, penetrometer deceleration will generally remain constant with penetration depth. Friction effects will be present but are generally small for cohesive soils and larger for granular soils. The unique character of penetrometer deceleration in granular and cohesive soils allows them to be identified through the measured penetrometer deceleration.
When the penetrometer impact velocity exceeds 30 m/s, the deceleration process is dominated by inertia for both granular and cohesive soils. At such high impact velocities, it becomes difficult, if not impossible, to identify the soil type from the penetrometer deceleration data. If there are no alternate means (e.g., remote sensing data) to identify soil type in the inertia driven regime, then it is necessary to estimate strength values (one value for granular soil and a second value for cohesive soil) using an analysis method that can separate the effects of inertia, friction, and strength. Since the strength magnitude will be for the deformation rate imposed by the high-velocity impact, it is necessary to correct the strength magnitude for rate effects, then a rate-corrected Cone Index measure (CI) can be estimated using a correlation to soil strength. If soil type can be identified using remote sensing data or by the addition of another sensor to the penetrometer, such as a pore pressure sensor, then the strength value for the identified soil can be calculated directly, using a method that accounts for inertia, friction, and strength.
At low-velocity impacts, inertia will still affect the deceleration process, but at a much reduced amount in comparison to friction and soil strength mechanisms. In this case, soil type can be identified as granular or cohesive directly from analysis of the penetrometer deceleration data. The association of penetrometer deceleration to soil strength has the possibility of being determined through direct correlation or the use of semi-physical models (to improve accuracy). Once strength is estimated it needs to be corrected for rate effects and then correlated with CI and/or CBR (California Bearing Ratio) for use in a vehicle mobility model. In order to decrease penetrometer velocity to ˜8 m/s, a number of methods can be employed, including deployable parachutes, but these have the disadvantage of being strongly affected by wind.